U.S. patent application number 13/955164 was filed with the patent office on 2015-02-05 for extreme ultraviolet (euv) mask, method of fabricating the euv mask and method of inspecting the euv mask.
This patent application is currently assigned to Taiwan Semiconductor Manufacturing Co., LTD.. The applicant listed for this patent is Taiwan Semiconductor Manufacturing Co., LTD.. Invention is credited to Jeng-Horng Chen, Yen-Cheng Lu, Chih-Tsung Shih, Anthony Yen, Shinn-Sheng Yu.
Application Number | 20150037712 13/955164 |
Document ID | / |
Family ID | 52427966 |
Filed Date | 2015-02-05 |
United States Patent
Application |
20150037712 |
Kind Code |
A1 |
Shih; Chih-Tsung ; et
al. |
February 5, 2015 |
Extreme Ultraviolet (EUV) Mask, Method Of Fabricating The EUV Mask
And Method Of Inspecting The EUV Mask
Abstract
An out-of-band (OoB) suppression layer is applied on a
reflective multiplayer (ML) coating, so as to avoid the OoB
reflection and to enhance the optical contrast at 13.5 nm A
material having a low reflectivity at wavelength of 193-257 nm, for
example, silicon carbide (SiC), is used as the OoB suppression
layer. A method of fabricating an EUV mask having the OoB
suppression layer and a method of inspecting an EUV mask having the
OoB suppression are also provided.
Inventors: |
Shih; Chih-Tsung; (Hsinchu
City, TW) ; Lu; Yen-Cheng; (New Taipei City, TW)
; Yu; Shinn-Sheng; (Hsinchu, TW) ; Chen;
Jeng-Horng; (Hsin-Chu, TW) ; Yen; Anthony;
(Hsinchu, TW) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Taiwan Semiconductor Manufacturing Co., LTD. |
Hsinchu |
|
TW |
|
|
Assignee: |
Taiwan Semiconductor Manufacturing
Co., LTD.
Hsinchu
TW
|
Family ID: |
52427966 |
Appl. No.: |
13/955164 |
Filed: |
July 31, 2013 |
Current U.S.
Class: |
430/5 |
Current CPC
Class: |
G03F 1/24 20130101; G03F
1/84 20130101; G03F 1/22 20130101; G03F 1/38 20130101; G03F 1/52
20130101 |
Class at
Publication: |
430/5 |
International
Class: |
G03F 1/22 20060101
G03F001/22; G03F 1/84 20060101 G03F001/84 |
Claims
1. An extreme ultraviolet (EUV) mask, comprising: a substrate; a
reflective multilayer (ML) coating over the substrate; an
out-of-band (OoB) suppression layer on the reflective ML coating;
and an absorber layer over the OoB suppression layer, wherein the
reflective ML coating is made of alternating layers of molybdenum
(Mo) and silicon (Si), wherein the number of the alternating layers
is in a range from about 30 pairs to about 60 pairs, and the OOB
suppression layer is made of at least a pair of Mo and silicon
carbide (SiC) layers.
2. The EUV mask of claim 1, wherein the absorber layer is made of a
material selected from the group consisting of TaBN, TaN and
CrN.
3. The EUV mask of claim 1, further comprising a buffer layer
between the OoB suppression layer and the absorber layer, wherein
the buffer layer acts as a capping layer.
4. The EUV mask of claim 3, wherein the buffer layer is made of
SiC.
5. The EUV mask of claim 3, wherein the buffer layer is made of a
material selected from the group consisting of silicon dioxide
(SiO.sub.2), silicon oxynitride (SiON), carbon (C), and ruthenium
(Ru).
6. The EUV mask of claim 4, wherein the buffer layer of SiC has a
thickness between 2 to 5 nm.
7. The EUV mask of claim 5, wherein the substrate is made of a low
thermal expansion material.
8. The EUV mask of claim 7, wherein the substrate is made of
quartz.
9. The EUV mask of claim 1, wherein the absorber layer comprises at
least one etch opening through which the reflective ML coating is
exposed.
10. A method of fabricating an extreme ultraviolet (EUV) mask
comprising: providing a substrate; depositing a reflective
multi-layer (ML) coating over the substrate, wherein the reflective
ML coating is made of alternating layers of molybdenum (Mo) and
silicon (Si), wherein the number of the alternating layers is in a
range from about 30 pairs to about 60 pairs; depositing an
out-of-band (OoB) suppression layer on the reflective ML coating,
wherein the OOB suppression layer is made of at least a pair of Mo
and silicon carbide (SiC) layers; and forming an absorber layer
over the OoB suppression layer.
11. The method of claim 10 further comprising forming a buffer
layer over the OoB suppression layer before forming the absorber
layer, and the buffer layer acts as a capping layer.
12. The method of claim 11, wherein forming the buffer layer is
forming the buffer layer of SiC.
13. The method of claim 12, wherein the buffer layer of SiC has a
thickness between 2 to 5 nm
14. The method of claim 10, further comprising: forming a resist
layer over the absorber layer, and patterning the resist layer to
form a trench with a trench width; etching through the absorber
layer, etching through the buffer layer to expose the OoB
suppression layer; and removing the resist layer.
15. The method of claim 14, wherein etching through the absorber
layer and the buffer layer comprise introducing a chlorine plasma,
an oxygen plasma, or both of the chlorine and oxygen plasma.
16. The method of claim 10, forming the absorber layer is forming
the absorber layer with a material selected from the group
consisting of TaBN, TaN and CrN.
17. The method of claim 10, depositing the buffer layer is
depositing the buffer with a material selected from the group
consisting of silicon dioxide (SiO.sub.2), silicon oxynitride
(SiON), carbon (C), and ruthenium (Ru).
18. A method of inspecting an EUV mask, comprising: providing the
EUV mask including a substrate, a reflective multilayer (ML)
coating over the substrate, an out-of-band (OoB) suppression layer
made of a pair of Mo and SiC layers on the reflective ML coating,
and a capping layer made of SiC on the OoB suppression layer;
irradiating the EUV mask to be inspected with inspection light to
illuminate a target region; detecting foreign matters from
diffusely reflected light; and cleaning and reusing the EUV
mask.
19. The method of claim 18, wherein a contrast of the inspection
light at wavelength of 193 nm is in a range of 0.65-0.90.
20. The method of claim 18, wherein the EUV mask is an attenuated
Phase Shift Mask (att-PSM).
Description
BACKGROUND
[0001] In the manufacture of integrated circuits (IC), patterns
representing different layers of the IC are fabricated using a
series of reusable photomasks ("masks") to transfer the design of
each layer of the IC onto a semiconductor substrate during the
manufacturing process in a photolithography process. These layers
are built up using a sequence of processes and resulted in
transistors and electrical circuits. However, as the IC sizes
continue to shrink, meeting accuracy requirements as well as
reliability in multiple layer fabrication has become increasingly
more difficult.
[0002] Photolithography uses an imaging system that directs
radiation onto the photomask and then projects a shrunken image of
the photomask onto a semiconductor wafer covered with photoresist.
The radiation used in the photolithography may be at any suitable
wavelength, with the resolution of the system increasing with
decreasing wavelength. Deep ultraviolet (DUV) light with a
radiation at a wavelength of 248 or 193 nanometers (nm) has been
widely used for exposure through a transmissive mask. However, with
the shrinkage in IC size, extreme ultraviolet (EUV) lithography
with a typical wavelength of 13.5 nm becomes one of the leading
technologies for 16 nm and smaller node device patterning.
[0003] An EUV mask utilized for the EUV lithography is a layered
structure including a Bragg mirror deposited on a substrate. On the
substrate, a reflective multilayer stack, which is formed by
sequentially stacking materials having different optical
properties, is used to achieve a high EUV light reflectance. The
pattern is formed from absorptive features or lines etched into the
EUV mask. The reflective multiplayer stack is a type of Bragg
reflector that reflects light at a selected wavelength through
constructive interference. The thicknesses of the alternating
layers are tuned to maximize the constructive interference (Bragg
reflection) of the EUV light reflected at each interface and to
minimize the overall absorption of the EUV light. The multiplayer
coating can achieve about 60 to 75% reflectivity at the peak
radiation wavelength. The EUV Lithography process may lack spectral
purity for its light sources, meaning the light sources may produce
undesirable out-of-band (OoB) radiation, i.e., radiation of an
undesirable bandwidth, for example, between 193 nanometers (nm) to
257 nm. Existing photoresist materials may be sensitive to the OoB
radiation and may absorb such radiation. This would result in
reduced contrast and hence degradation of imaging performance.
[0004] On the other hand, the EUV masks require frequent cleaning
to reduce or eliminate defects during the optical lithography
operation. The cleaning is typically performed at an elevated
temperature to enable and/or enhance the efficiency of the cleaning
chemistry. In addition, during use the masks are inadvertently
heated through exposure with extreme ultraviolet light. In this
regard, the mask is frequently exposed to temperatures above
ambient during the masks lifecycle and is used at temperatures
exceeding ambient during normal operation. Consequently, these
conditions can cause several types of chemical diffusion and
chemical reactions within the multilayer stack of the Bragg
mirror.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] Embodiments of the present disclosure are best understood
from the following detailed description when read with the
accompanying figures. It is emphasized that, in accordance with the
standard practice in the industry, various features are not drawn
to scale. In fact, the dimensions of the various features may be
arbitrarily increased or reduced for clarity of discussion.
[0006] FIG. 1 is a block diagram of a photolithography imaging
system that uses a mask in processing a wafer.
[0007] FIG. 2 is a cross-sectional view schematically illustrating
an EUV mask according to various embodiments of the present
disclosure.
[0008] FIG. 3A is a cross-sectional view schematically illustrating
mask according to various embodiments of the present
disclosure.
[0009] FIG. 3B is a cross-sectional view schematically illustrating
an EUV mask according to various embodiments of the present
disclosure.
[0010] FIG. 3C is a cross-sectional view schematically illustrating
an EUV mask according to various embodiments of the present
disclosure.
[0011] FIG. 3D are diagrammatic cross-sectional side views of the
EUV mask of FIG. 2A at various stages of fabrication according to
various embodiments of the present disclosure.
[0012] FIG. 4 is a flowchart illustrating a method of fabricating
an EUV mask according to various embodiments of the present
disclosure.
[0013] FIG. 5 is a flowchart illustrating a method of inspecting an
EUV mask according to various embodiments of the present
disclosure.
DETAILED DESCRIPTION
[0014] It is to be understood that the following disclosure
provides many different embodiments, or examples, for implementing
different features of the disclosure. Specific examples of
components and arrangements are described below to simplify the
present disclosure. These are, of course, merely examples and are
not intended to be limiting. Moreover, the formation of a first
feature over or on a second feature in the description that follows
may include embodiments in which the first and second features are
formed in direct contact, and may also include embodiments in which
additional features may be formed interposing the first and second
features, such that the first and second features may not be in
direct contact. Various features may be arbitrarily drawn in
different scales for the sake of simplicity and clarity.
[0015] The singular forms "a," "an" and "the" used herein include
plural referents unless the context clearly dictates otherwise.
Therefore, reference to, for example, a gate stack includes
embodiments having two or more such gate stacks, unless the context
clearly indicates otherwise. Reference throughout this
specification to "one embodiment" or "an embodiment" means that a
particular feature, structure, or characteristic described in
connection with the embodiment is included in at least one
embodiment of the present disclosure. Therefore, the appearances of
the phrases "in one embodiment" or "in an embodiment" in various
places throughout this specification are not necessarily all
referring to the same embodiment. Further, the particular features,
structures, or characteristics may be combined in any suitable
manner in one or more embodiments. It should be appreciated that
the following figures are not drawn to scale; rather, these figures
are intended for illustration.
[0016] The EUV lithography is an exposure technique using EUV
light. The EUV light refers a ray having a wavelength in a soft
X-ray region or a vacuum ultraviolet ray region. Specifically, the
EUV light has a wavelength of about 10 to 20 nm, particularly about
13.5 nm.+-.0.3 nm. In the EUV lithography, illumination is cast on
the EUV mask at an angle, e.g., 5.degree. relative to the axis
perpendicular to a plane of the mask. Challenges exist in an optics
system of the EUV lithography for transferring a pattern to the
wafer, including the optics deformation, contamination, source
stability, dose uniformity, shot noise, flare (stray lights),
optical contrast, etc.
[0017] As to the optical contrast of the pattern formed from the
EUV mask at 13.5 nm, the reflective multiplayer stack also reflects
radiation of bandwidths that are out of a desired bandwidth of the
EUV band. For example, radiation having a bandwidth in a range
between about 193-247 nm is considered undesirable Out-of-Band
(OoB) radiation for EUV lithography processes. The OoB radiation is
included in light generated from a light source of EUV exposure
apparatus. As a result, the absorption of such OoB radiation
results in reduced optical contrast and degradation of imaging
performance of conventional photoresist materials. Exposure of the
EUV photoresist to OoB radiation typically results in unwanted
background exposure of the resist called "flare." Flare among other
things hurts the resolution of the resist, reducing contrast with
respect to unexposed areas, and compromising the ability to etch
patterns of sufficiently small sizes.
[0018] According to various embodiments of the present disclosure,
an OoB suppression layer is applied on a reflective multiplayer
(ML) coating, so as to avoid the OoB reflection and thus enhance
the optical contrast at 13.5 nm. A material having a low
reflectivity at wavelength of 193-257 nm, for example silicon
carbide (SiC), is used as the OoB suppression layer. In some
embodiments, the OoB suppression layer is a composite layer having
two layers, a SiC layer and a Mo layer. According to various
embodiments of the present disclosure, a buffer layer made of SiC
can be deposited over the reflective ML coating or the OoB
suppression layer to enhance the optical contrast at 13.5 nm.
[0019] According to various embodiments of the present disclosure,
a method of inspecting an EUV mask is provided. In the EUV
lithography process, the EUV masks require frequent cleaning to
reduce or eliminate defects during the optical lithography
operation. In the cleaning process, the mask is frequently exposed
to temperatures above ambient during the masks lifecycle and is
used at temperatures exceeding ambient during normal operation.
Consequently, defects occur within the reflective ML coating and
degrade the performance of the EUV mask. In various embodiments of
the present disclosure, the EUV mask including the OoB suppression
layer also has an improved optical contrast at a radiation of
wavelength at 193 nm. Because of the enhanced optical contrast at
wavelength at 193 nm, the method according to the various
embodiments of the present disclosure provides better detecting
efficacy of the defects or particles on the EUV mask during the
cleaning process or usage operation. Further, the OoB suppression
layer of SiC has a better resistance than the conventional
materials (e.g., Si) to attacks of the chemicals used in the EUV
lithography process or the cleaning process.
[0020] Photolithography uses an imaging system that directs
radiation onto a mask having a pattern and then projects a reduced
image of that mask onto a semiconductor wafer covered with
photoresist. The radiation used in photolithography may be at any
suitable wavelength, with the resolution of the system increasing
with decreasing wavelength. The ability to print smaller features
onto the semiconductor wafer improves as the resolution increases.
Concerning EUV lithography, it is based on exposure with the
portion of the electromagnetic spectrum having a wavelength of
10-15 nanometers. An EUV step-and-scan tool may have a 4-mirror,
4.times.-reduction projection system with a 0.10 Numerical Aperture
(NA). Exposure is accomplished by stepping fields over a wafer and
scanning the EUV mask across each field. Various types of masks
used in photolithography include such as binary mask, alternating
phase-shift mask, and attenuated phase-shift mask (att-PSM), as
well as various hybrid mask types. The EUV mask may be fabricated
by exposing and developing the photoresist layer of the blank
substrate to form a photoresist pattern and by etching the
absorption layer and the buffer layer using the photoresist pattern
as an etch mask to form an absorption layer pattern. If the
absorption layer pattern is formed to have a critical dimension
(CD), for example, of a size that is different from the design CD
value, it may be difficult to compensate the CD of the absorption
layer pattern. A CD of 50-70 MD may be achieved with a depth of
focus (DOF) of about 1 micrometer (um). Alternatively, a 6-mirror,
4X-reduction projection system may be applied with a 0.25 NA to
print a smaller CD of 20-30 nm, at the expense of a reduced DOF.
Other tool designs with a 5X-or a 6X-reduction projection system
may also be used for EUV lithography.
[0021] Referring to FIG. 1, an EUV lithography imaging system 100
includes a radiation source 110, a condenser optics section 120, a
projection optics section 130, a mask stage 140, and a wafer stage
150. The radiation source 110 may be any source able to produce
radiation in the EUV wavelength range. One example of a suitable
radiation source 110 is creates a plasma when a laser illuminates a
gas, such as a supersonic jet of xenon gas. As another example, a
suitable radiation source 110 may be use bending magnets and
undulators associated with synchrotrons. As a further example, a
suitable radiation source 110 may be use discharge sources, which
have the potential to provide adequate power in the desired
wavelength range. EUV radiation is strongly absorbed in virtually
all transmissive materials, including gases and glasses. To
minimize unwanted absorption, EUV imaging is carried out in near
vacuum.
[0022] The condenser optics section 120 brings the radiation from
the source 110 to the mask stage 140. In the EUV lithography
imaging system 100, the condenser optics are reflective because EUV
radiation is strongly absorbed in traditionally transmissive
materials such as lenses, which may be used in traditional
photolithography imaging systems. Accordingly, the condenser optics
section 120 includes condenser reflectors or mirrors 125 that
collect and focus the radiation from the source 110 onto the mask
stage 140. Any number of condenser mirrors 125 may be used, such
as, for example, the four shown in FIG. 1.
[0023] The mask stage 140 includes a transport stage 146 that scans
a mask 142. In the EUV lithography imaging system 100, the mask 142
is reflective because EUV radiation is strongly absorbed in most
materials such as transmissive photomasks that are used in
traditional photolithography imaging systems.
[0024] The projection optics section 130 reduces the image from the
mask 140 in the mask stage 140 and forms the image onto wafer 152
in the wafer stage 150. In the EUV lithography imaging system 100,
the projection optics are reflective because of the absorption
associated with EUV radiation. Accordingly, the projection optics
section 130 includes reflectors or mirrors 135 that project
radiation reflected from the mask 140 onto the wafer. The
reflectance spectrum of the mask 142 may be matched to that of the
mirrors in the projection optics section 130. The term "projection
optics" used herein should be broadly interpreted as encompassing
any type of projection system, including refractive, reflective,
catadioptric, magnetic, electromagnetic and electrostatic optical
systems, or any combination thereof, as appropriate for the
exposure radiation being used.
[0025] The wafer stage 150 includes a transport stage 156 that
scans a semiconductor wafer 152 in synchrony with the mask 142 and
steps the wafer 152 into a position to accept a next image from the
mask 142. During operation, a semiconductor wafer 152 mounted to
the transport stage 156. The projection optics convey the radiation
light with a pattern in its cross-section to create a pattern in a
target portion of the wafer 152. It should be noted that the
pattern conveyed to the radiation light may not exactly correspond
to the desired pattern in the target portion of the wafer, for
example if the pattern includes phase-shifting features or shadows.
Generally, the pattern conveyed to the radiation light will
correspond to a particular functional layer in a device being
created in a target portion of the wafer 152, such as an IC.
[0026] FIG. 2 is a schematic cross-sectional view of an EUV mask
200, having a patterned absorber layer 240 according to various
embodiments of the present disclosure. The EUV mask 200 includes a
substrate 210, a reflective multilayer (ML) coating 220 for
reflecting EUV light, an out-of-band (OoB) layer consisting of a
first layer 226 and a second layer 224, and a buffer layer 230. In
various embodiments, a capping layer 228 made of SiC may be
deposited on the OoB suppression layer 224/226 prior to the forming
of the buffer layer 230. In some embodiments, the buffer layer 230
acts as a capping layer, such that the capping layer of 228 is the
same layer of the buffer layer 230. According to embodiments of the
present disclosure, the patterned absorber layer 240 is a dual
layer of consisting of a first layer 242 and a second layer
244.
[0027] During the EUV lithography process, up to about 40% of the
EUV light is absorbed by the EUV mask. Thermal expansion caused by
the heating leads to a large image distortion that may exceed the
error tolerance. Low thermal expansion material (LTEM) has been
used as the substrate material for the substrate of the EUV masks.
The substrate 210 may have a low thermal expansion coefficient (for
example, the thermal expansion coefficient within a temperature
range of from 19.degree. C. to 27.degree. C. is
0.+-.1.0.times.10.sup.-7/.degree. C. In various embodiments, the
thermal expansion coefficient is 0.+-.0.3.times.10.sup.-7/.degree.
C., 0.+-.0.2.times.10.sup.-7/.degree. C.,
0.+-.0.1.times.10.sup.-7/.degree. C., or
0.+-.0.05.times.10.sup.-7/.degree. C. A glass having a low thermal
expansion coefficient, such as a .beta. quartz may be used as the
substrate 210. Further, a film such as a stress correcting film
(not shown) may be formed on the substrate 210.
[0028] The reflective ML coating 220 of the EUV mask is used to
achieve a high EUV light reflectance. The reflective ML coating 220
is a type of Bragg reflector that reflects light at a selected
wavelength through constructive interference. The selection of
materials in the ML coating 220 depends on the radiation wavelength
(.lamda.) to be reflected. Each layer of the ML coating 220 has a
thickness of about one quarter of .lamda.. In particular, the
thickness of the respective layers of the ML coating 220 depends on
the radiation wavelength and the incidence angle of the radiation
light. For EUV, the .lamda. is 13.5 nm and the incidence angle is
about 5 degrees. Using many reflecting film pairs, over 60%
reflectance for light having a wavelength in the vicinity of 13.5
nm is achieved. The thicknesses of the alternating layers are tuned
to maximize the constructive interference (Bragg reflection) of the
EUV light reflected at each interface and to minimize the overall
absorption of the EUV light. The ML coating 220 can achieve about
60-75% reflectivity at the peak radiation wavelength. The
reflective ML coating 220 is formed by sequentially stacking
materials 222/224 having different optical properties. The Bragg
reflection occurs at the interface of the materials 222/224. The
reflectivity of the reflective ML coating 220 is proportional to
the square of the difference between the refractive indexes (real
parts of complex refractive indexes) of the two materials 222/224
that are alternately stacked. In addition, the wavelength and
maximum reflectivity of the reflected EUV light are determined
based upon the kinds of the materials in 222/224. In various
embodiments, the reflective ML coating 220 has 30 pairs to about 60
pairs of alternating layers of a low index of refraction material
224 and a high index of refraction material 222. For example, 40
pairs of the alternative layers 222/224 of the first ML film 320
are deposited in which the high index of refraction material 222
may be formed from about 2.8 nm thick Molybdenum (Mo) while the low
index of refraction material 224 may be formed from about 4.1 nm
thick Silicon (Si).
[0029] According to various embodiments of the present disclosure,
the OoB layer consisting of a first layer 226 and a second layer
224 is formed on a top surface of the reflective ML coating 220.
The first layer 226 is made of a material having a low reflectivity
of the radiation at wavelength of 193-257 nm, and a low refraction
index than that of the material for the second layer 224. The term
"low'" reflectivity refers to the material having a value of
reflectivity to the radiation at 193-257 nm lower than that of
materials used in the reflective ML coating 220. In embodiments,
the first layer 226 may be made of SiC and the second layer 224 be
made of Mo. Accordingly, the EUV mask of FIG. 2 reflects less OoB
light and thus improves optical contrast in the EUV region.
[0030] A buffer layer 230, such as about 11 nm thick Ruthenium
(Ru), may be formed over the top surface of the OoB suppression
layer 224/226. in some embodiments, a capping layer 228 made of SiC
may be deposited on the OoB suppression layer 224/226 prior to
forming the buffer layer 230. The Mo layer in either the reflective
ML coating 220 or the OoB suppression layer can become oxidized
under ambient conditions. The capping layer 228 of SiC prevents
oxidation of the Mo layer. In certain embodiments, the capping
layer 228 of SiC may be the buffer layer 230; that is, the buffer
layer 230 is the same layer of the capping layer 228 of SiC and
acts as the capping layer. In specific embodiments, the capping
layer 228 of SiC has a thickness of 2-7 nm, and for example, a
thickness of 3 nm.
[0031] According to various embodiments of the present disclosure,
a patterned absorber layer 240 is formed over the reflective ML
coating 220. In various embodiments, the absorber layer 240 has a
thickness d in a range of 30-70 nm. In some embodiments, the
absorber layer 240 is a dual-layer stack having a first layer 242
and a second layer 244. In various embodiments, more than one
dual-layer stack may be deposited over the reflective ML coating
220.
[0032] FIGS. 3A-3D are the cross-sectional side views of the EUV
mask 200 of FIG. 2A at various stages of manufacture according to
various embodiments of the present disclosure. For reasons of
simplicity, FIGS. 3A-3D may only illustrate a part of the EUV
mask.
[0033] hr FIG. 3A, a substrate 310 with a low defect level and a
smooth surface is used as the starting material for the EUV mask
200 in the present disclosure. The substrate 310 has a low
coefficient of thermal expansion (CTE). In some embodiments, the
substrate 310 is a glass or glass-ceramic material. For example,
the substrate 310 may be formed of .beta.-quartz,
[0034] Referring to FIG. 3B, a reflective ML coating 320 is formed
over the substrate 310. The reflective ML coating 320 has about
30-60 pairs of alternating layers of a low index of refraction
material 324 and a high index of refraction material 322. In some
embodiments, the reflective ML coating 320 has 40 pairs of the
alternative layers 322/324. A high index of refraction material 322
includes elements with high atomic number which tend to scatter EUV
light. A low index of refraction material 324 includes elements
with low atomic number which tend to transmit EUV light. The
reflective ML coating 320 is formed over the substrate 310 by using
ion beam deposition (IBD) or DC magnetron sputtering. The thickness
uniformity should be better than 0.8% across the substrate 310. IBD
results in less perturbation and fewer defects in the upper surface
of the reflective ML coating 320 because the deposition conditions
can usually be optimized to smooth over any defect on the substrate
310. In various embodiments, 40 pairs of the alternative layers
322/324 of the ML coating 320 are deposited in which the high index
of refraction material 322 may be formed from about 3 nm thick Mo
while the low index of refraction material 324 may be formed from
about 4 nm thick Si. For example, the high index of refraction
material 322 may be formed from about 2.8 nm thick Mo while the low
index of refraction material 324 may be formed from about 4.1 nm
thick Si.
[0035] In various embodiments, in fabricating an EUV mask, a
substrate 310 may be provided having the reflective ML coating 320
thereon. In this case of the substrate 310 already having the
reflective ML coating 320, the operation in FIG. 3B may be omitted
in the method of fabricating the EUV mask according to the
embodiments of the present disclosure.
[0036] As shown in FIG. 3C, an OoB suppression layer consisting of
a first layer 326 and a second layer 324 is deposited on the
reflective ML layer 320. The first layer 326 is made of a material
having a low reflectivity of the radiation at wavelength of 193-257
nm, and a low refraction index than that of the material for the
second layer 324. In embodiments, the first layer 326 may be made
of SiC and the second layer 324 be made of Mo. In various
embodiments, the first layer 326 and the second layer 324 may be
formed by using ion beam deposition (IBD) or DC magnetron
sputtering.
[0037] In some embodiments, a capping layer 328 made of SiC may be
deposited on the OoB suppression layer 324/326. Because the Mo
layer in either the reflective ML coating 320 or the OoB
suppression. layer can become oxidized under ambient conditions,
the capping, layer 328 of SiC prevents oxidation of the Mo layer.
In certain embodiments, the capping layer 328 of SiC has a
thickness of 2-7 nm, and for example, a thickness of 3 nm. In some
embodiments, the capping layer 328 is made of Ruthenium (Ru). In
various embodiments, the first layer 326 and the second layer 324
may be formed by using ion beam deposition (IBD) or DC magnetron
sputtering
[0038] The buffer layer 330 is formed over the OoB suppression
layer 324/326 or the capping layer 328. The buffer layer 330 may
have a thickness of about 20-60 nm. The buffer layer may be formed
from silicon dioxide (SiO.sub.2) or a silicon (Si) layer, In
various embodiments, the buffer layer may a Ru capping layer formed
at the top of the ML coating 320 to prevent oxidation of Mo by
exposure to the environment. The buffer layer 330 may be low
temperature oxide (LTO) as SiO.sub.2, or other materials, such as
silicon oxynitride (SiOxNy) or carbon (C). The buffer layer 330 may
act later as an etch stop layer for patterning of the overlying
absorber 340 formed in the following operation. In some
embodiments, the capping layer 328 of SiC may be the buffer layer
330; that is, the buffer layer 330 is the same layer of the capping
layer 328 of SiC and acts as the capping layer. Furthermore, the
buffer layer 330 may also serve later as a sacrificial layer for
focused ion beam (FIB) repair of defects in the absorber 340. The
buffer layer 330 may be deposited by a suitable process such as
magnetron sputtering and ion beam sputtering.
[0039] Referring to FIG. 3D, an absorber layer 340 is formed over
the buffer layer 330 or the capping layer 328 according to various
embodiments of the present disclosure. In embodiments, the absorber
layer 340 has a total thickness d raging from 30-70 nm. The
absorber layer 340 may be deposited by RF sputtering, DC
sputtering, ion beam deposition (IBD) or atomic layer chemical
vapor deposition (ALD). In various embodiments, shown in FIG. 3D,
the absorber layer 340 is a dual-layer stack including a first
layer 342 and a second layer 344 [not shown in FIG. 3D] made of
highly absorptive materials to the radiation of wavelength at 13.5
nm. Patterning the absorber layer 340 includes forming a
photoresist pattern over the absorber layer 340 in an absorption
region, etching the absorber layer 340 by using the photoresist
pattern as an etch mask to form an absorber pattern, and removing
the photoresist pattern. In particular, the absorber layer 340 may
be covered with a radiation-sensitive layer, such as photoresist,
that is coated, exposed, and developed with a desired pattern. The
photoresist pattern has a thickness of about 160-640 nm. As
desired, a chemically-amplified resist (CAR) may be used. Depending
on the photoresist pattern used, exposure is performed on an
electron beam (e-beam) writer or a laser writer. Reactive ion etch
may be used. For example, an absorber layer 340 may be dry etched
with a gas that contains chlorine, such as Cl.sub.2 or BCl.sub.3,
or with a gas that contains fluorine, such as NF.sub.3. Argon (Ar)
may be used as a carrier gas. In some cases, oxygen (O.sub.2) may
also be included as carrier. The etch rate and the etch selectivity
depend on the etchant gas, etchant flow rate, power, pressure, and
substrate temperature. The buffer layer 330 may serve as an etch
stop layer to help achieve a good etch profile in the overlying
absorber layer 340. The buffer layer 330 protects the underlying
reflective ML coating 320 from damage during etch of the absorber
layer 340.
[0040] FIG. 4 is a flowchart illustrating a method of fabricating a
method of fabricating an EUV mask according to various embodiments
of the present disclosure. The operations are explained in the
cross-sectional side views of a portion of the EUV mask 200 from
FIGS. 3A to 3D at various fabrication stages according to various
embodiments of the present disclosure. It is understood that FIGS.
3A-3D have been simplified for a better understanding of the
inventive concepts of the present disclosure.
[0041] In FIG. 3A, a substrate 310 is provided in operation 402.
Referring to FIG. 3A, the substrate 310 is made of a material
having a low coefficient of thermal expansion (CTE). For example,
the substrate 310 may be formed of .beta.-quartz.
[0042] Referring to the operation 404, a reflective ML coating 320
is deposited over the substrate 310. In FIG. 3B, the reflective ML
coating 320 has about 30-60 pairs of alternating layers of a low
index of refraction material 322 and a high index of refraction
material 324. In embodiments, the reflective ML coating 320 has 40
pairs of the alternative layers 322/324.
[0043] As various embodiments, in fabricating an EUV mask, a
substrate 310 may be provided having the reflective ML coating 320
thereon. In this case of the substrate 310 with the reflective ML
coating 320, the operation 404 in FIG. 4 may be omitted in the
method of fabricating the EUV mask according to the embodiments of
the present disclosure.
[0044] In embodiments, the method of fabricating the EUV mask
further includes an operation 406 of depositing an OoB suppression
layer on the ML coating in FIG. 4. Referring to FIG. 3C, the OoB
suppression layer consists of a first layer 326 and a second layer
324. In embodiments, the first layer 326 is made of a material
having a low reflectivity of the radiation at wavelength of 193-257
nm, and a low refraction index than that of the material for the
second layer 324. In embodiments, the first layer 326 may be made
of SiC and the second layer 324 be made of Mo. Accordingly, the OoB
layer exhibit a reduced OoB light in view of 13.5 nm and an
improved optical contrast as far as the radiation at 193-257 nm
wavelength is concerned.
[0045] Still referring to FIG. 3C, in embodiments, a capping layer
328 made of SiC may be deposited on the OoB suppression layer
324/326. Because the Mo layer in either the reflective ML coating
320 or the OoB suppression layer can be oxidized under ambient
conditions, the capping layer 328 of SiC prevents oxidation of the
Mo layer. In embodiments, the capping layer 328 of SiC has a
thickness of 2-7 nm, and for example, a thickness of 3 nm. In
various embodiments, a buffer layer 330 is formed over the OoB
suppression layer 324/326 or the capping layer 328, The buffer
layer 330 may have a thickness of about 20-60 nm. The buffer layer
may be formed from silicon dioxide (SiO.sub.2) or a silicon (Si)
layer. In various embodiments, the buffer layer may a Ru capping
layer formed at the top of the ML coating 320 to prevent oxidation
of Mo by exposure to the environment. The buffer layer 330 may be
low temperature oxide (LTO) as SiO.sub.2, or other materials, such
as silicon oxynitride (SiOxNy) or carbon (C). The buffer layer 330
may act later as an etch stop layer for patterning of the overlying
absorber 340 formed in the following operation. In embodiments, the
capping 3 of SiC may be the buffer layer 330; that is, the buffer
layer 330 is the same layer of the capping layer 328 of SIC and
acts as the capping layer.
[0046] Referring to the operation 408 of FIG. 4, the absorber layer
340 is formed on the top surface of the OoB suppression layer. In
various embodiments, the absorber 340 may be formed on the buffer
layer 330 or the capping layer 328. The desired pattern on an EUV
mask is defined by selectively removing an absorber layer to
uncover portions of an underlying mirror coated on a substrate.
According to various embodiments of the present disclosure, the
method of fabricating the EUV mask further includes an operation of
forming a resist layer over the absorber layer, patterning the
resist layer to form a trench with a trench width, and etching
through the absorber layer 340 to expose the reflective ML coating
and removing the resist layer. As a result, the patterned absorber
layer 340 of FIG. 2 is formed accordingly.
[0047] FIG. 5 is a flowchart illustrating a method of inspecting an
EUV mask according to various embodiments of the present
disclosure. In operation 502, the EUV mask is provided with a
substrate, a reflective multilayer (ML) coating over the substrate,
an out-of-band (OoB) suppression layer made of, for example, a pair
of Mo and SiC layers on the reflective ML coating, and a capping
layer made of SiC on the OoB suppression layer. In operation 504,
the EUV mask is radiated with, for example, a wavelength of 193 nm,
for inspection of a target region. In operation 506, foreign
matters are detected from diffusely reflected light. Further, the
EUV mask is cleaned at suitable process to be reused for the next
EUV lithography process.
[0048] According to various embodiments, a contrast of the
inspection light at wavelength of 193 nm is in a range of
0.65-0.90. In embodiments, wherein the EUV mask is an attenuated
Phase Shift Mask (att-PSM). [This is a fragment. What are you
trying to say?] With the method of inspecting the EUV mask
according to the embodiments of the present disclosure, the EUV
mask including the OoB suppression layer also has an improved
optical contrast at a radiation of wavelength at 193 nm. Because of
the enhanced optical contrast at wavelength at 193 nm, the EUV mask
having an OoB suppression layer can be inspected with better
detecting efficacy of the defects on or in the EUV mask. Further,
the OoB suppression layer of SiC has a better resistance than the
conventional materials (e.g., Si) to attacks of the chemicals used
in the EUV lithography process or the cleaning process.
[0049] The foregoing has outlined features of several embodiments
so that those skilled in the art may better understand the detailed
description that follows. Those skilled in the art should
appreciate that they may readily use the present disclosure as a
basis for designing or modifying other processes and structures for
carrying out the same purposes and/or achieving the same advantages
of the embodiments introduced herein. Those skilled in the art
should also realize that such equivalent constructions do not
depart from the spirit and scope of the present disclosure, and
that they may make various changes, substitutions and alterations
herein without departing from the spirit and scope of the present
disclosure.
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